1. Field of the Invention
The present invention relates to apparatus for determining external dimensional relationships of three-dimensional objects for ascertaining distortion thereof. In particular, the invention relates to apparatus for determining such dimensional relationships and distortion data in irradiated nuclear fuel assemblies.
A nuclear fuel assembly typically comprises an array of elongated fuel rods and control rods engaged with and held in position by a plurality of grids longitudinally spaced-apart between top and bottom nozzles. In a typical reactor core, there may be between 160 and 190 such fuel assemblies, each fuel assembly including approximately 200 fuel rods, and each being held in place by retaining springs.
Prior to use, each fuel assembly is substantially straight, the fuel pins and associated control rods for the fuel assembly all being parallel to each other and perpendicular to the top and bottom nozzles and to the grids. However, each reactor cycle results in a certain amount of distortion of the fuel assembly. This distortion includes "bow", "twist" and "tilt". "Bow" is defined as the deviation of the centroid of each grid from a straight line connecting the centroids of the top and bottom nozzles. "Twist" is defined as the angular rotation of each grid with respect to a zero reference plane defined by one face of the top nozzle. "Tilt" is defined as the distance between the centroid of the bottom nozzle and a vertical line passing through the centroid of the top nozzle. After each refueling cycle, fuel assemblies are moved to different locations in the reactor core, new fuel assemblies being added along the outside or periphery of the core and being gradually moved inwardly toward the center. After about three refueling cycles, a fuel assembly will reach the center of the core and become spent, these spent assemblies being removed during the next refueling. Each fuel assembly has locating holes in the bottom nozzle for receiving locating pins in the bottom core plate, accurately to position the fuel assembly in the reactor core. The above-described distortion can prevent accurate alignment of the bottom nozzle with these locating pins during repositioning of the fuel assemblies.
Furthermore, because of newer fuel assembly design utilizing thinner walls and higher spring forces for retaining the fuel assembly in place, bow is becoming even more of a problem than heretofore. Deformation of as little as one-half inch can prevent accurate resetting of a fuel assembly in a new location in the reactor core.
2. Description of the Prior Art
Previously, bow has been measured by hanging a measurement plate with horizontal rulings in front of the fuel assembly and reading the bow against the ruled markings, and thereby estimating the amount of bow in the fuel assembly. Alternatively, a closed circuit TV camera may be moved along the periphery of the fuel assembly, the camera movement being monitored to provide an estimate of a fuel assembly bow. Both of these prior techniques are time consuming and relatively inaccurate. Heretofore, no means has been available for making adequate tilt and twist measurements, and such measurements have generally not been attempted.